Hopping mice do it differently: Mice with minute testes, misshapen sperm and thin, spiny penises may shed light on what makes most males the way they are

By BILL BREED

AMONG Australia’s famous marsupials live many unsung native rodents.
Arguably the most bizarre are the hopping mice, members of the genus Notomys.
Dwelling largely in the arid zone, these animals have big eyes and ears,
a long brush-tipped tail and extremely long hind limbs and feet that enable
them to hop rapidly across the open sand. But what makes the hopping mice
especially peculiar – and peculiarly interesting – is their strange reproductive
anatomy.

A sexually mature male hopping mouse has remarkably small testes. The
biologists who first found and described these animals never commented on
this fact, but perhaps they thought that all the males they collected were
reproductively inactive, even though they collected some pregnant females
as well. Our research has concentrated on a captive colony of spinifex hopping
mice at the University of Adelaide. Usually, rodents in the murid group,
which includes laboratory rats or mice and even most native Australian species,
have large and conspicuous testes carried in scrotal sacs. But in these
hopping mice, the testes can be a twentieth the size of those in most other
species of similar body size.

Most rodents also have large ‘accessory’ sex glands. Often the most
noteworthy are the seminal vesicles, whose secretions form the large copulatory
‘plug’ which the males deposit in the vagina after ejaculation to facilitate
the passage of sperm into the uterus. In the spinifex hopping mice, the
situation is quite different. They have only one large accessory sex gland,
the bilobed ventral prostate: all the other glands, including the seminal
vesicles and coagulating glands, are minute. Not surprisingly, the males
do not …

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